This invention relates to diagnostic ultrasonic imaging, and, more particularly, to a system and method for displaying tissue perfusion and other time-varying parameters.
Ultrasonic diagnostic imaging systems are capable of imaging and measuring the physiology within the body in a completely noninvasive manner. Ultrasonic waves are transmitted into the body from the surface of the skin and are reflected from tissue and cells within the body. The reflected echoes are received by an ultrasonic transducer and processed to produce an image or measurement of blood flow. Diagnosis is thereby possible with no invasion of the body of the patient.
Materials known as ultrasonic contrast agents can be introduced into the body to enhance ultrasonic diagnosis. Contrast agents are substances that strongly reflect ultrasonic waves, returning echoes which may be clearly distinguished from those returned by blood and tissue. One class of substances which has been found to be especially useful as an ultrasonic contrast agent is gases, in the form of tiny bubbles called microbubbles. Microbubbles strongly backscatter ultrasound in the body, thereby allowing tissues and blood containing the microbubbles to be readily detectable through special ultrasonic processing.
Although microbubbles can be simply very small bubbles of a suitable gas, microbubbles can also be very small bubbles of gas coated with a thin biodegradable coating or shell. These coated microbubbles typically have diameters between 0.1 and 4.0 microns and a specific density about {fraction (1/10)} of the density of water. Coated microbubbles are normally suspended in an aqueous solution for infusion into the blood stream. Coated microbubbles have the advantage of being stable in the body for a significant period of time, as the shells serve to protect the gases of the microbubbles from diffusion into the bloodstream. The size of the microbubbles may be chosen to enable the microbubbles to pass through capillary beds in the body. Therefore, microbubble contrast agents can be used for imaging the body""s vascularized tissues, such as the walls of the heart, since the contrast agent can be injected into the bloodstream and will pass through veins, arteries and capillaries with the blood supply until filtered from the blood stream in the lungs, kidneys and liver.
Although coated microbubbles can survive in the body for an extended period, they can also be selectively destroyed. More specifically, at moderately high sound pressure amplitudes, acoustic pressure waves can cause the shells of coated microbubbles to rupture, freeing the bubbles to behave as non-coated microbubbles by quickly diffusing into the bloodstream. U.S. Pat. No. 5,813,613 to Averkiou, et al., which is incorporated herein by reference, discloses a technique for using the destruction of microbubbles as described above to provide a measure of tissue perfusion. Basically, the Averkiou, et al. technique involves transmitting a high intensity ultrasonic pulse to a selected sample volume in the body, thereby destroying the microbubbles at that location. After the microbubbles in the sample volume have been destroyed, the blood that contained the destroyed microbubbles flows out of tissues at that location, and new blood containing microbubbles reperfuses these tissues. After a given period of reperfusion another high intensity pulse is transmitted with the received echoes indicating the microbubble concentration at the sample volume after the given period of reperfusion. This pulse destroys the microbubbles a second time, and a different reperfusion period is allowed to pass and another high intensity pulse transmitted to determine the microbubble concentration after the different period of reperfusion. The cycle is repeated for a plurality of reperfusion period and the measurements thus taken are plotted to produce a reperfusion curve showing the rate of reperfusion of the tissue at the sample volume.
While this technique is effective for measuring the perfusion rate and producing a reperfusion curve for a specific sample volume, it would be desirable to be able to make and display the results of perfusion measurements for a large region of tissue and not just a particular sample volume location. Such a capability would enable the rapid diagnosis the perfusion rate of a significant region of tissue such as the myocardium, enabling the clinician to quickly identify small regions of tissue where perfusion is problematic due to ischemia or other bloodflow conditions.
A method and system displays a parametric contrast image of the perfusion rate of anatomy in an image, depicting the rate of reperfusion for a significant area or volume of tissue in the body simultaneously. A single static parametric perfusion image may be produced, or a sequence of parametric perfusion images produced to illustrate changes in the perfusion rate over time. Ungated or gated images may be used, enabling the parametric display to be keyed to specific phases of the heart cycle. Multiple parameters can be combined in a single parametric image, if desired. The inventive technique may be used with bolus contrast injections or with a continuous infusion of the contrast agent.